The present invention relates in general to nonlinear integrated and fiber optics and more specifically to completely optical switches and optical transistors and can be used in fiber-optic optical communications, in optical logical schemes and in other fields, where all-optical switching, amplification, controlling and modulation optical radiation is need.
The method of self-switching of unidirectional distributively coupled waves (UDCW) is known [A. A. Maier, xe2x80x9cThe method of signal switching in tunnelly-coupled optical waveguidesxe2x80x9d, USSR PATENT No1152397 (September 1982); Byull. Izobret. (46) 300 (1988); A. A. Maier. Physics-Uspekhi vol.38 (No 9) pp.991-1029 (1995)]. The method consists in a sharp change of a ratio of intensities (and phases) of the waves at output of tunnel-coupled optical waveguides (TCOW) caused by a small variation of intensities or phases of these waves at the input of the TCOW. Due to given method the earlier unknown class of optical transistors was suggested. The important advantage of the fiber optical transistor is convenience of its junction with optical fiber communication lines. The phenomenon of self-switching is accompanied by auto-synchronization of waves, i.e. alignment of phases of waves at the output of TCOW in so-called midpoint of self-switching [(A. A. Maier. Physics-Uspekhi v.38 (No 9) 991-1029 (1995)].
As one from perspective variants of the optical transistor the so-called discrete optical transistor was proposed [A. A. Maier. Sov. J. Quantum Electron. v.17, p.1013 (1987)], in which as pump the sequence of super-short pulses is used.
If the dispersion is essential in fiber-optic waveguide, that takes place in long fiber-optic waveguides, the optimum shape for supershort pulses is soliton shape.
It is known that, while propagating along the fiber-optic waveguide, even over large distances, the soliton pulses do not diffuse (broaden), saving the form sech(t), since for them the nonlinear compression is compensated by dispersion diffusing. Therefore solitons are perspective for transfer of recordly large information contents.
The fact that solitons can be switched completely as a unit, thus providing complete self-switching, i.e., high effective gain for the discrete optical transistor [A. A. Maier Sov. J. Quantum Electron. v.17, p.1013 (1987)], is of even greater importance for us. It is explained by the fact that a soliton propagating along the fiber optic waveguide retains a uniform phase temporal profile, i.e., for all points of the soliton, its phase is nearly the same and depends only on the longitudinal coordinate z. Self-switching occurs near the self-switching midpoint M, corresponding to the unit modulus of the elliptic function, through which the output intensity is expressed. At this point, the output wave amplitudes and phases at the output of the zeroth and the first waveguides are equal, and the characteristic rate of change (i.e., the sensitivity to small variations of input powers and phases) is maximum.
The closest to the proposed method is the method (S. Trillo, S. Wabnitz, E. M. Wright, G. I. Stegeman. Optics Lett. 1988, 13, 672-674) of switching of pulses close to the second-order solitons (when input intensities a002=3.63, a102=0) in cubic-nonlinear TCOW.
Defects of this method are the rather high energy of solitons fed into the waveguides, and also small sharpness and depth of the switching.
Technical aim of the invention is a decreasing of energy of solitons fed into the waveguides, and also increase of sharpness and depth of switching, and gain of optical transistor.
In the first variant of the method for switching, amplification and modulation of unidirectional distributively coupled pulses and waves they feed radiation in the form of pulses with different maximum intensity |ak02| into the input of at least one of tunnel-coupled optical waveguides, having nonlinearity and the second-order dispersion,
the put task is solved by that
as the pulses they use fundamental solitons or fundamental soliton-like pulses with amplitude and shape near to that of fundamental solitons, maximum intensity |ak02| of which lies in the range from 0.6IM to 1.4IM, here IM is critical intensity, thereto IM=(2÷8)K/|xcex8|, where
ak0 is input pulse amplitude in soliton normalization, k is number of waveguide,
K is coefficient of tunnel coupling in soliton normalization averaged over the length of tunnel coupling of the waveguides,
xcex8 is arithmetic average nonlinear coefficient of the tunnel-coupled optical waveguides in soliton normalization.
As a rule, IM=(5÷7)K/|xcex8|. In particular cases IM=(5.5÷6.5)K/|xcex8|, and IM=(5.9÷6.1)K/|xcex8|.
As a rule, as the pulses fed into input of waveguide they use fundamental solitons or fundamental soliton-like pulses with amplitude and shape near to that of fundamental solitons, maximum intensity |ak02| of which is in the range from 0.9IM to 1.1IM. In special cases said maximum intensity |ak02| lies in the range from 0.99IM to 1.01IM. In particular cases the |ak02| is in the range from 0.995IM to 1.005IM.
The nearness of pulse amplitude to the amplitude of the fundamental soliton consists in that they use pulses with amplitude 0.5 less than ak0 less than 1.5.
In particular the length of tunnel coupling of the waveguides is more or equal to half of the beat length of transfer of radiation power between the waveguides in linear regime. The beat length of transfer of radiation power between the waveguides in linear regime is meant as the length of the tunnel-coupling, at which the transfer of energy from zero waveguide to the first waveguide takes place, provided that radiation is fed only into the zeroth waveguide and nonlinear factor is equal to zero, i.e. for feeding optical radiation, square of intensity of which is by the order of magnitude less than square of the critical intensity. Very sharp switching takes place with the length of tunnel coupling of the waveguides is more than or equals three lengths of transfer of radiation power between the waveguides in linear regime.
In particular case, all said pulses are fed into the input of only one of said tunnel-coupled optical waveguides.
In particular case, the tunnel-coupled optical waveguides are made as dual-core fiber optic waveguides.
In the second variant of the method, consisting in that they feed pump optical radiation in the form of pulses with maximum intensity |ak02| into the input of at least one of tunnel-coupled optical waveguides, having cubic nonlinearity and dispersion of the second order,
the put task is solved by that
into the input of another or of the same waveguide they feed radiation with variable intensity and/or maximum intensity and/or phase, thereto maximum intensity |A52| of this radiation is at least in ten times less than maximum intensity |ak02|, and as the pulses fed into the input of zero waveguide they use fundamental solitons or fundamental soliton-like pulses with amplitude and shape near to that of fundamental solitons, maximum intensity of which lies in the range from 0.6IM to 1.4IM, here IM is critical intensity, thereto IM=(2÷8)K/|xcex8|, where:
ak0 is input pulse amplitude in soliton normalization, k is a number of waveguide,
K is coefficient of tunnel coupling of the waveguides in soliton normalization averaged over the length of tunnel coupling,
xcex8 is arithmetic average nonlinear coefficient of two waveguides in soliton normalization.
As a rule, IM=(5÷7)K/|xcex8|. In particular cases IM=(5.5÷6.5)K/|xcex8|, and IM=(5.9÷6.1)K/|xcex8|.
The nearness of the pulse amplitude to the amplitude of fundamental soliton consists in that they use pulses with input real amplitude 0.5 less than ak0 less than 1.5.
As a rule, the maximum intensity |ak02| is in the range from 0.9IM to 1.1IM . In particular cases it is in the range from 0.99IM to 1.01IM and even in the range from 0.995IM to 1.005IM.
In different cases, maximum intensity |A62| of the radiation with variable parameter is at least in 1000 times less than maximum intensity |ak02| of the pump pulses, and in at least 104 times less than maximum intensity |ak02|.
As a rule, the length of tunnel coupling is more than or equal to half of the beat length of transfer of radiation power between the waveguides in linear regime. Very sharp switching takes place with the length of tunnel coupling of the waveguides is more than or equals the three lengths of transfer of radiation power between the waveguides in linear regime.
In particular case, all said pump pulses and all signal optical radiation with variable input intensity and/or variable maximum input intensity and/or variable phase are fed into the input of only one of said tunnel-coupled optical waveguides.
In particular, the tunnel-coupled optical waveguides are made as dual-core fiber optic waveguides.
In particular case as the radiation having variable intensity, fed into the input of the first waveguide, they use fundamental solitons or nearby to them pulses.
In the third variant of the method, consisting in that they feed optical radiation in the form of pulses with different and/or variable maximum intensity |ak02| or with different and/or variable phase into the input of one of tunnel-coupled optical waveguides, having nonlinearity and dispersion of the second order,
the put task is solved by that
into the input of another or the same waveguide they additionally feed optical radiation with the different or the same phase or maximum intensity, thereto as the pulses they use fundamental solitons or fundamental soliton-like pulses with amplitude and shape near to that of fundamental solitons, thereto maximum intensity of these pulses is larger than IM/4 or equal to IM/4, where IM is the critical intensity, thereto IM=(2÷8)K/|xcex8|, where:
ak0 is input pulse amplitude in soliton normalization, k is a number of waveguide,
K is coefficient of tunnel coupling between the waveguides in soliton normalization averaged over the length of the tunnel coupling of the waveguides,
xcex8 is arithmetic average nonlinear coefficient of two waveguides in soliton normalization.
As a rule, IM=(5÷7)K/|xcex8|. In particular cases IM=(5.5÷6.5)K/|xcex8|, and IM=(5.9÷6.1)K/|xcex8|.
The nearness of pulse amplitude to the amplitude of fundamental soliton consists in that they use pulses with real input amplitude 0.5 less than ak0 less than 1.5 in soliton normalization.
In particular case the length of tunnel coupling is more than or equal to the beat length of transfer of radiation power between the waveguides in linear regime. In particular the length of tunnel coupling is more than or equals two lengths of the energy transfer in linear regime.
In special case all said pulses are fed into the input of only one of the tunnel-coupled optical waveguides.
In the fourth variant, consisting in that they feed optical radiation in the form of pulses having different maximum intensity into input of one of tunnel-coupled waveguides, which have cubic nonlinearity and the second-order dispersion,
the put task is solved by that
a part of the pulses has maximum intensity |ak02, so that |ak02|2 greater than 13IM2, where IM is the critical intensity, another part of pulses has maximum intensity |ak02, so that |ak02|2 less than IM2/13, thereto as the pulses they use fundamental solitons or fundamental soliton-like pulses close to them in shape or amplitude, thereto 2K/|xcex8| less than IM less than 8K/|xcex8|, where
a00 is input amplitude of a pulse in soliton normalization,
K is coefficient of the tunnel coupling between the waveguides in soliton normalization averaged over the length of tunnel coupling,
xcex8 is arithmetic average nonlinear coefficient of two waveguides in soliton normalization.
As a rule, IM=(5÷7)K/|xcex8|. In particular cases IM=(5.5÷6.5)K/|xcex8|, and IM=(5.9÷6.1)K/|xcex8|.
The nearness of pulse amplitude to the amplitude of fundamental soliton consists in that they use input pulses with real amplitude 0.5 less than ak0 less than 1.5, thereto the square of maximum intensity of the pulses is at least by the order of magnitude more than square of the critical intensity.
In particular case the length of tunnel coupling between the waveguides is equal to odd number of transfers of power between the waveguides in linear regime. As a rule the length of tunnel coupling is more or equals the length of transfer of radiation power between the waveguides in linear regime. In special case it is larger than or equal to three lengths of transfer of the optical radiation power between the waveguides in linear regime.
In particular case the tunnel-coupled optical waveguides are made as dual-core fiber optic waveguides.
In special case all said pulses are fed into the input of only one of the tunnel-coupled optical waveguides.
In the fifth variant of the method the put task is solved by that optical radiation with variable parameter is fed into nonlinear optical waveguide having the second-order dispersion, the waveguide is made as birefringent, the fed radiation consists of pump pulses and signal pulses with variable intensity and/or phase, thereto the polarizations of the pump pulses and signal pulses are mutually orthogonal, and polarization of one of radiation is directed along  less than  less than fast greater than  greater than  or  less than  less than slow greater than  greater than  axis of the optical waveguide, or at the angle to this axis, which does not exceed xcfx80/10, thereto the input power is chosen from the condition ay02xe2x89xa7|xcex1|/|xcex8| or ax02xe2x89xa7|xcex1|/|xcex8|, where ay0 or ax0 is amplitude of pump pulse, xcex1 is normalized birefringence of the waveguide, xcex8 is normalized nonlinear coefficient of the waveguide, thereto as the pulses the fundamental solitons or nearby to them pulses are used.
In particular, to provide high contrast (depth) of the switching the angle between polarization vector of the pump radiation and  less than  less than fast greater than  greater than  or  less than  less than slow greater than  greater than  axis does not exceed such the angle at which normalized coefficient of linear coupling K=0.05.
In preferable embodiment the waveguide is cubic nonlinear.
In preferable case the nonlinear waveguide is made as fiber optic waveguide.
In another particular case the nonlinear-optical waveguide is made on the basis of semiconductor layered MQW-type structure with alternating layers, containing at least two hetero-transitions, thereto in particular case the semiconductor layered MQW-type structure is made as alternating layers GaAs/AlxGa1xe2x88x92xAs, or InxGa1xe2x88x92xAs/InP, or In1xe2x88x92xGaxAsyP1xe2x88x92y/In1xe2x88x92x.Gax.Asy.P1xe2x88x92y., where xxe2x89xa0xxe2x80x2 and/or yxe2x89xa0yxe2x80x2, or CdSe1xe2x88x92xSx/CdSe or InAs1xe2x88x92xSbx/InAs, or PbSxSe1xe2x88x92x/PbSe, or GexSi1xe2x88x92x/Si.
In another particular case the intensity amplitude of signal pulses is at least by two orders of magnitude less than intensity amplitude of pump pulses.
The most gain is reached under normalized birefringence of the waveguide satisfying to inequalities 0.01xe2x89xa6xcex1xe2x89xa60.7, the difference between phases of pump and signal radiation at the input satisfying to inequalities 0xe2x89xa6"psgr"xe2x89xa6xcfx80, and amplitude of pump pulses (at the input) satisfying to inequalities 0.7xe2x89xa6a less than 1.45.
In particular case the largest gain is reached when the normalized birefringence of the waveguide is in the range 0.07xe2x89xa6xcex1xe2x89xa60.4, the difference between phases of pump and signal radiation at the input is in the range xcfx80/3xe2x88x92xcfx80/5xe2x89xa6"psgr"xe2x89xa6xcfx80/3+xcfx80/5, and amplitude of pump pulses at the input is in the range 1.05xe2x89xa6axe2x89xa61.3.
In the sixth variant of the method for switching amplification and modulation of optical radiations consisting in that they feed optical radiation in the form of pulses with different maximum intensity, to the input of one of TCOW (e.g., zero), having nonlinearity and second-order dispersion,
the put task is solved by that
the tunnel-coupled optical waveguides have quadratic nonlinearity,
the input normalized complex amplitudes of the pulses correspond to formulas
Ajk(z=0)=xcex1jkexp(ixcfx86jk)/cos hxcexc[(txe2x88x92tjk,d)/xcfx84jk,p],
or Ajk(z=0)=xcex1jkexp(ixcfx86jk)/exp[xe2x88x92(txe2x88x92tjk,d)xcexc/xcfx84jk,pxcexc],
where 1.5xe2x89xa6xcexcxe2x89xa62.5, k=0,1 is number of the optical waveguide, j=1 corresponds to frequency xcfx89, j=2 corresponds to frequency 2xcfx89, t is time, xcfx84jk,p is duration of input pulse at the input of the k-th optical waveguide at frequency jxcfx89, tjk,d is time delay of the pulse at the input of the k-th optical waveguide at frequency jxcfx89, xcfx86jk is input phase of the pulse with frequency jxcfx89 at the input of the k-th waveguide, ajk is input real amplitude of the pulse, i.e. it is amplitude module of the pulse with frequency jxcfx89 at the input of the k-th waveguide,
the pulses are fed at frequencies xcfx89 and 2xcfx89 into the input of one of tunnel-coupled optical waveguides, or into inputs of the different tunnel-coupled optical waveguides,
the input normalized real amplitudes ajk of the fed pulses are chosen to satisfy to at least one pair of following pairs of inequalities: a10xe2x89xa72 and a20xe2x89xa72, a11xe2x89xa72 and a21xe2x89xa72, a11xe2x89xa72 and a20xe2x89xa72, a21xe2x89xa72 and a10xe2x89xa72,
under this the switching of the pulses from one waveguide to another is done by change of amplitude a10 and/or amplitude a11, and/or amplitude a20, and/or amplitude a21, or phase xcfx8610 and/or phase xcfx8611, and/or phase xcfx8620, and/or phase xcfx8621 of fed pulses at the input of at least one of said optical waveguides with at least one of said frequencies.
In the most preferable case xcexc=2. As a rule, tjk,d less than 3xcfx84p; more typically tjk,d less than xcfx84p; for preferable embodiment tjk,d less than  less than xcfx84p, and they can consider xcfx84jk,d=0.
As a rule, a difference of xcfx84jk,p from the average quadratic duration xcfx84p of the fed pulses is not more than xcfx84p. Typically, xcfx84jk,p=xcfx84p, i.e. all the input pulses have the same input duration.
In the preferable embodiment said amplitudes ajk are chosen to satisfy to at least one pair of the following pairs of inequalities: 3xe2x89xa6a10 less than 9{square root over (2)} and 3xe2x89xa6a20xe2x89xa69{square root over (2)}, 3xe2x89xa6a11xe2x89xa69{square root over (2)} and 3xe2x89xa6a21xe2x89xa69{square root over (2)}, 3xe2x89xa6a11xe2x89xa69{square root over (2)} and 3xe2x89xa6a20xe2x89xa69{square root over (2)}, 3xe2x89xa6a21xe2x89xa69{square root over (2)} and 3xe2x89xa6a10xe2x89xa69{square root over (2)}.
Under this, as a rule, variation of amplitude ajk, causing switching of pulses from one waveguide to another, does not exceed 0.2 of maximum from the values ajk, or variation of phase xcfx86jk, causing switching of pulses from one waveguide to another, does not exceed 0.2xcfx80.
Under this, as a rule, the length of tunnel coupling of the waveguides is more or equal to a half of minimum from the beat lengths at frequencies xcfx89 and 2xcfx89 in linear regime.
In particular cases the tunnel-coupled optical waveguides are made in the form dual-core fiber optic waveguide or on the basis of KTP crystal, or on the basis of semiconductor layered quantum-well structure, or on the basis of crystal fiber from ferro-organic materials with large quadratic nonlinearity.
Besides as a rule phase-matching condition is fulfilled.
For effective embodiment of the sixth variant of the method, and also for saving a shape of pulse under the dispersion of the second order as a rule the following inequalities |xcex94|xe2x89xa610K1, |xcex1j|xe2x89xa610K1, |("khgr"1xe2x88x92"khgr"0)/"khgr"|xe2x89xa610K1, 0.08xe2x89xa6|D1k/D2k|xe2x89xa612 are to be satisfied, where xcex94=(xcex940+xcex941)/2, xcex94k=xcex22kxe2x88x92xcex21k is the difference between effective refractive indexes at frequencies 2xcfx89 and xcfx89 in the k-th waveguide, xcex1j=xcex2j1xe2x88x92xcex2j0 is difference between effective refractive indexes of waveguides  less than  less than 1 greater than  greater than  and  less than  less than 0 greater than  greater than  at frequency jxcfx89, K1, is coefficient of tunnel coupling at frequency xcfx89, Djk is coefficient of second-order dispersion in the k-th waveguide at frequency jxcfx89, "khgr"=(|"khgr"0|+|"khgr"1|)/2, "khgr"k is quadratic-nonlinear coefficient of k-th waveguide.
In the seventh variant of the method consisting in that they feed optical radiation to the input of nonlinear tunnel-coupled optical waveguides, switching of optical radiation from one of said waveguides to another is accomplished by variation of one of parameter of the radiation,
the put task is solved by that
the tunnel-coupled optical waveguides have quadratic nonlinearity,
the fed radiation contains optical waves with frequencies xcfx89 and 2xcfx89, which are fed into the input of one optical waveguide or into inputs of different optical waveguides,
input normalized amplitudes xcfx81jk(z=0) of the fed waves are chosen to satisfy to at least one pair of following pairs of inequalities: xcfx8110(z=0)xe2x89xa72 and xcfx8120(z=0)xe2x89xa72, xcfx8111(z=0)xe2x89xa72 and xcfx8121(z=0)xe2x89xa72, xcfx8111(z=0)xe2x89xa72 and xcfx8120(z=0)xe2x89xa72, xcfx8121(z=0)xe2x89xa72 and xcfx8110(z=0)xe2x89xa72, where k=0,1 is a number of the optical waveguide, j=1 corresponds to frequency xcfx89, j=2 corresponds to frequency 2xcfx89,
under this the switching is fulfilled by variation of amplitude xcfx8110(z=0) and/or amplitude xcfx8111(z=0), and/or amplitude xcfx8120(z=0), and/or amplitude xcfx8121(z=0), or phase xcfx8610(z=0), and/or phased xcfx8611(z=0), and/or phase xcfx8620(z=0), and/or phase xcfx8621(z=0) of fed waves at the input of at least one of said optical waveguides for at least one of said frequencies.
As a rule, said normalized input real amplitudes are chosen to satisfy to at least one pair of the following pairs of inequalities: 3xe2x89xa6xcfx8110(z=0)xe2x89xa69{square root over (2)} and 3xe2x89xa6xcfx8120(z=0)xe2x89xa69{square root over (2)}, 3xe2x89xa6xcfx8111(z=0)xe2x89xa69{square root over (2)} and 3xe2x89xa6xcfx8121(z=0)xe2x89xa69{square root over (2)}, 3xe2x89xa6xcfx8111(z=0)xe2x89xa69{square root over (2)} and 3xe2x89xa6xcfx8120(z=0)xe2x89xa69{square root over (2)}, 3xe2x89xa6xcfx8121(z=0)xe2x89xa69{square root over (2)} and 3xe2x89xa6xcfx8110(z=0)xe2x89xa69{square root over (2)}.
As a rule, the change of said normalized real input amplitude xcfx81jk(z=0), causing the switching of optical radiation from one waveguide to another does not exceed 0.2 of maximum from values of the input amplitudes xcfx81jk(z=0), or change of input phase xcfx86jk(z=0), causing the switching of optical radiation from one waveguide to another does not exceed 0.2xcfx80.
Under this a length of tunnel coupling of the waveguides is more than or equal to half of minimal value from the beat lengths at frequencies xcfx89 and 2xcfx89 in linear regime.
In particular cases the tunnel-coupled optical waveguides are made in the form of dual-core fiber-optic waveguide or on the basis of KTP crystal, or on the basis of semiconductor layered quantum-well structure, or on the basis of crystal fiber from ferro-organic materials with large quadratic nonlinearity.
Besides, as a rule, phase-matching condition is fulfilled.
As a rule, for accomplishment effective switching it is needed |("khgr"1xe2x88x92"khgr"0)/"khgr"|xe2x89xa610K1, where K1 is coefficient of tunnel coupling at frequency xcfx89, "khgr"=(|"khgr"0|+|"khgr"1)/2, "khgr"k is a quadratic-nonlinear coefficient of the k-th waveguide.